A material made of a Cu alloy on which a Sn-plating layer is formed has been widely used for electrical contact components. In recent years, in order to miniaturize and lighten a contact component, an ordinary steel sheet or a stainless steel sheet, which has a material strength higher than that of a Cu alloy even though the sheet has a small sheet thickness, has been used as a base material for Sn-plating (for example, Patent Literatures 1 to 4). The stainless steel sheet has a material strength higher than that of the ordinary steel sheet and is excellent in corrosion resistance of a cut edge portion after cutting processing for forming a contact component. Therefore, the stainless steel sheet is suitable for the base material for Sn-plating.
With regard to an electrical contact material having a Sn-plating layer, due to internal stress (compressive stress) generated in the Sn-plating layer, for example, electrodeposition stress at Sn-plating, stress generated at the formation of a Sn alloy which is formed by diffusion of a metal from the base material metal and the like, a needle-like single crystal which is called as a whisker is generated from the Sn-plating layer. When the whisker is generated, there is a possibility that some troubles are caused by a short circuit. Therefore, a material having whisker resistance has been required for an electrical contact material. Conventionally, in order to suppress the generation of a whisker in a Cu alloy material, various counter-measures have been studied. In recent years, in order to suppress the generation of a whisker from a Sn-plating layer in stainless steel as well as the Cu alloy material, there has been studied to form a Ni-plating layer under the Sn-plating layer, or carry out a reflow treatment of the stainless steel.
However, the mechanism of generation of a whisker from the Sn-plating layer has not yet been elucidated, and the mechanism of generation of a whisker varies depending on a kind of a base material being used, a thickness of a plating layer, a surrounding environment and the like.
A whisker has hitherto been investigated under various testing environments and by various testing methods (see, for example, Non-patent Literature 1). It has been confirmed that a whisker is generated by compressive stress which is generated in a Sn-plating layer due to oxidation of the Sn-plating layer, difference in coefficient of thermal expansion between a base material and an underlying plating layer and the like under an environment such as a high-temperature and high-humidity environment, an environment where a temperature is rapidly changed and a low temperature condition and a high temperature condition are repeated in a short period of time.
In recent years, it has been desired to develop a stainless steel sheet having whisker resistance under various environments, which has a Sn-plating layer excellent in adhesion to the stainless steel sheet which is used as a base material of an electrical contact material.